Fluidized bed combustors, whether circulating, bubbling, pressurized or at atmospheric pressure, have the ability to capture sulphur dioxide in situ by use of a sulphur absorbent which is typically either a calcitic limestone or dolomite. Other sorbents have been considered but price and availability have favoured calcium based sorbents especially where the sorbent is not to be regenerated.
Generally fluidized bed combustors involve sulphur capture via the following overall reactions:CaCO3=CaO+CO2, ΔH=182.1 kJ/gmolCa O+SO2+½O2=CaSO4 ΔH=481.4 kJ/gmolThe first reaction is endothermic and the second exothermic. In pressurized systems, calcination of CaCO3 does not occur. Here the basic reaction isCa.CO3+SO2+½O2=CaSO4+CO2 The reaction is exothermic.If dolomitic stone is used, the reactions areCaMg(CO3)2=CaCO3MgO+CO2 CaCO3MgO+SO2+½O2=CaSO4MgO+CO2 
The sulphation reaction is far from quantitative: typically 30-45% CaO conversion is obtained. In fact, between two and three times as much CaCO3 must be calcined as is converted to CaSO4. This relatively low utilization of limestone is one of the major limitations of the technology.
Because so much more limestone must be used than is effectively converted to CaSO4, the ash from such combustors contains a substantial amount of calcium that has not been reacted to calcium sulphate. It is known that it is desirable to reactivate the ash so that the calcium can be made available to be turned into calcium sulphate. To do otherwise means that the combustor must be fed with a larger amount of limestone which impairs its efficiency.
Although the desire to reactivate ash is well-known, there remains a need to develop other mechanisms for reactivating such ash. Accordingly, it is an object of this invention to provide a system for reactivating ash so that it may take place in the sulphation reaction for the purpose of reducing sulphur dioxide emissions from such combustors.
Coal, when burnt, often involves a coal washing process. Typically in the coal washing process the particles of coal are reduced in size to pieces in the order of two inches in diameter or at least ¼ of an inch and larger. Fines from such a process may be recovered by way of flotation and by drying the coal. Typically, wet fine coal has not been considered to be of economic value. It is extremely expensive to dry wet fine coal to recover the coal and the fine powder recovered in such processes is difficult to handle. Accordingly, there has been a widespread practice of simply dumping the fine wet coal in tailing ponds and the like. Although it is known that the fine coal exists in such ponds, there has not heretofore been a satisfactory economic process for recovering the coal and using that fine coal as a fuel source.
Ultimately it would be desirable to develop a process to enable fine wet coal to be utilized as a feed source.
In accordance with another aspect of this invention, fine wet coal can be utilized as a fuel source by combining the fine wet coal with ash.